The small signal theory analysis and optimization design of high efficiency broadband TWTs

Broadband helix traveling wave tube (TWT), because of its wide bandwidth and high gain, has potential applications in electronic countermeasure (ECM) systems and space exploration. It´s a challenge to obtain high efficiency TWT over a multi-octave band, since its fundamental efficiency is degraded and harmonic power content is enhanced at lower band frequencies and the efficiency at high frequencies deteriorates because of insufficient interaction impedance and large distributing loss. The fundamental method of the design of an ultra-broadband TWT usually uses two-section output circuit: the first output section using unloaded circuit is to make the growing wave gain occur only at lower end of the band; the second output section using a loaded helix circuit is designed to be in synchronism with the RF wave at all frequency. In this paper, this fundamental method was analyzed using Pierce small signal theory, and then the special helix pitch profile using only negative dispersion slow wave structure (SWS) was shaped to satisfy the design request. The helix pitch profile was optimized to obtain a higher electronic efficiency using genetic algorithm at high frequency (18GHz).